Electrolytic copper plating solutions are used for many industrial applications. For example, electrolytic copper plating solutions are used in the automotive industry to deposit base layers for subsequently applied decorative and corrosion protection coatings. They are also used in the electronic industry, especially during fabrication of printed circuit boards. During circuit fabrication, copper is electroplated over selected portions of the surface of the printed circuit board and onto the walls of through holes passing between the surfaces of the circuit board base material. The walls of the through holes are metallized to provide conductivity between the circuit layers of the printed circuit board. Thus, in many printed circuit board and semiconductor fabrication processes, electroplating has been adopted by industry as the primary deposition means for copper metallization.
Many commercial copper electroplating solutions comprise aqueous solutions of copper sulfate, copper fluorine-borate, copper methane sulfonate, copper cyanide, or copper pyrophosphate along with various organic additives that improve the properties of the deposited copper.
The most widely used copper plating electrolyte is based on an aqueous solution of copper sulfate, an acid electrolyte such as sulfuric acid, and various plating additives. Commonly used additives for copper metallization include inhibitors/suppressors, brighteners/accelerators, and/or levelers. The brighteners (or accelerators) change the nucleation process by accelerating the charge transfer process at the copper interface, providing active growth sites, while the suppressors (or inhibitors) adsorb at the cathode surface uniformly, increasing the deposition over-potential. In other words, one of the key functions of the organic additives is to level the deposit by suppressing the electrodeposition rate at protruding areas in the substrate surface and/or by accelerating the electrodeposition rate in recessed areas. The adsorption and inhibition may be further enhanced by the presence of halogen ions.
These organic additives must also be closely controlled (e.g., in the low parts per million range) in order to attain the desired deposit properties and surface morphology.
Brighteners typically comprise sulfur-containing organic compounds, and may also incorporate functional groups, such as described for example in U.S. Pat. No. 5,252,196 to Sonnenberg et al., the subject matter of which is herein incorporated by reference in its entirety. Organic polymers are commonly used as the suppressor additives for copper electroplating. Leveling agents include polyamines and reaction products of an amine with an alkylene oxide and epihalohydrins as well as dye compounds such as phenazinium compounds, as described for example in U.S. patent publication No. US 2007/0108062 to Brunner et al., U.S. patent publication No. US 2006/0226021 to Brunner et al., and U.S. patent publication No. US 2004/0231995 to Murao, the subject matter of each of which is incorporated herein by reference in its entirety.
In most instances, the recommended working temperature for the sulfuric acid plating solution does not exceed about 80° F. (27° C.), and more typically, plating is carried out at room temperature, or about 70-74° F. (21-23° C.). Because these copper plating electrolytes are designed for use at room temperature, they are not generally suited for plating through holes at elevated temperatures. In many instances, the brighteners undergo chemical changes at elevated temperatures and are no longer effective for copper plating. In other instances, the levelers used in combination with the wetter/suppressor additives in the solution present issues leading to the deposition of dull, rough layers, especially inside the through holes. The thermal characteristics of the copper layer deposited at elevated temperatures are also adversely affected and reliability performance is decreased. Failure can also occur during the soldering operation that follows plating.
Printed circuit board fabrication has dramatically increased over the past few years in geographic areas with hot climates. In order to maintain the desired temperature in these areas, chillers or other cooling means are generally needed. Thus, it is desirable to simplify the process in these areas to eliminate the need for chillers or other cooling means and still obtain a desired plating deposit.
To that end, the present invention relates generally to an improved acid copper plating bath composition containing various plating bath additive that provides a desired plating deposit, especially at elevated temperatures.